Astronomers getting their first close-up glimpse of a giant asteroid about to whiz by Earth found a surprise bonus rock. A smaller moon asteroid is circling the larger space rock,an unusual but not unheard of space phenomenon.

The larger 1.7 mile-wide rock named Asteroid 1998 QE2 will be the closest to Earth on Friday at 4.59 pm EDT. Don’t worry,though. It will still be 3.6 million miles away. NASA scientist Paul Chodas said it’s one of the larger asteroids to swing by Earth and is the size of the space rock that wiped out the dinosaurs.

The smaller rock was discovered Wednesday night by astronomers using radar to look at QE2. The moon asteroid is about 2,000 feet wide. That’s about average for such near-Earth objects.